Both alkaline and acid processes have been used for etching aluminum. The alkaline process for etching aluminum in its simplest form utilizes sodium hydroxide solutions. In time, the concentration of dissolved aluminum in the etch tank increases to the point where the walls and bottom of the tank are covered with a hard crust, mainly of sodium aluminate. Further, the copper and manganese constituents of most structural aluminum alloys form a black deposit on the etched aluminum. Compositions containing chromic acid or nitric acid as the principal oxidizers must be used to remove this black deposit, commonly known as "smut."
More recent etchant/deoxidizer compositions that eliminate environmentally controlled chromic acid contain mixtures of water, ferric sulfate, nitric acid, and fluoride chemicals such as hydrofluoric acid and its salts. The acid process for etching aluminum eliminates the requirement of redissolving the black deposit of copper and manganese salts. As in the alkaline process, however, in time the concentration of dissolved aluminum in this acid medium builds up until an aluminum oxyfluoride precipitate forms.
Thus, there is a need for an etchant/deoxidizer which reduces or eliminates the need for constantly removing crystals from the bottom and sides of tanks.